Joseph Westphal, until recently the ambassador to Saudi Arabia, saw plenty of turbulence. Oil prices plummeted, there was a royal leadership succession, and the Saudi government launched an ambitious economic reform program.

Westphal also navigated a relationship under great strain. The U.S. spearheaded a historic agreement that eased sanctions on Saudi-rival Iran in order to end its nuclear program. International human rights groups condemned the kingdom's execution of a Shia cleric and its intervention in the Yemen civil war. Late in President Obama's term, Congress overrode a veto of controversial legislation, known as JASTA, to clear the way for lawsuits seeking damages from the kingdom for the 9/11 terrorist attacks.

Yet, Saudi Arabia — the biggest foreign customer for American military hardware — still sees the U.S. as a vital ally for its economic future and against threats from the Islamic State (ISIL), al-Qaida, and Iran, its most potent foe.

The former academic leader and U.S. Army undersecretary relinquished ambassadorship when Donald Trump became president. He spoke with Defense News earlier this month about the defense ties, the turbulence and the kingdom's outlook on President Trump.

How did the Iran deal take the U.S.-Saudi relationship to a difficult place? 

Their objection was not about the nuclear piece itself. They don't want a nuclear Iran more than any more than we, Israel, or anybody else wants it. They saw some goodness in the ability to negotiate a deal to maybe take that piece out of the occasion. Where I think they really were stressed with us was that they wanted the U.S., in the negotiations, to push very hard for Iran to stop what it was doing in Syria, stop its meddling in Yemen and other parts of the Middle East. The White House clearly wasn't going to go in that direction.

What caused anxiety between the two countries was that, in negotiating this — and I wasn't part of the negotiations with Iran — but everything was kept so close, so that even I, as the ambassador, had a hard time briefing the Saudis on what was going on, because we weren't privy to it. I urged a number of times to provide more information. But I think those who were negotiating found that could put the deal at risk. Whoever was right or wrong on this, history will tell. Did that permanently damage the relationship or cause a serious problem? No.

The president made an attempt to show that despite differences on how to approach issues like the Iran negotiations, Saudi Arabia was still critical and important to the United States. The president made four visits to Saudi Arabia. There were a lot of phone calls between the president and the king. The president received the crown prince and the deputy crown prince in the Oval Office and typically, that's reserved for heads of state.

President Obama made clear the U.S. would not be part of the coalition fighting in Yemen, but it would provide ISR, intelligence and refueling. Was there a philosophical reluctance to engaging? 

The president clearly, philosophically, was not going to commit to any engagement of that magnitude. And to be a party to supporting a campaign in Yemen, whether you're providing intelligence, or ISR, or whatever else kind of support you're providing, you're going to be accused or seen as being part of this war and part of this coalition.

There were some realities people don't know. During the early part of this, a lot of our assets were still committed in Afghanistan and still are. U.S. Central Command didn't have the capability to — even if the president had said so — to provide assets the Saudis wanted and, perhaps, needed in this situation. So, when they did [bomb] the hospital or bomb on [a Yemeni] condolence ceremony for civilians, the stress was, "Why are we allowing them to do this?" Well we're really not allowing them. We're not part of this coalition. We don't provide them the targeting, we don't provide them the intel. So, it then created a real problem for everybody.

Did the U.S. feel pressured to assist in the Yemen campaign because the relationship was strained by the Iran deal?

It has nothing to do with that. Iran has a lot to do with it because obviously, the Houthis and [ousted Yemeni President] Ali Abdullah Saleh's forces have been in cahoots with Iran. These rockets that they keep firing into Saudi Arabia and have fired on our ships. That's Iranian technology. Those are Iranians supporting and often doing that. So, what Saudi Arabia couldn't understand is, we've got Iran on our border, helping and supporting this. But again, the president wasn't willing to go beyond a very limited level of support.

To the extent that there were strained relations, was there an impact on security cooperation, military relationships, opportunities for the defense industry?

At the very end, some people at the National Security Council decided they wanted to send a message to Saudi Arabia about some of the humanitarian concerns with the bombing campaign. By doing an assessment of the kind of military sales programs we had with them — and they produced a policy, weeks before the administration exited office, to say that we're going to delay or hold off on the sale of precision-guided munitions. Which, in my view, was the wrong end because PGM's are the very things that they need in order to avoid collateral damage. It was not, "You're never going to get them," it was a delay.

We have a very robust program for the F-15, and probably several weeks before this decision by the White House, I went to an event to receive four brand new F-15s for Saudi Arabia, which are the most sophisticated airplanes out there. There wasn't any attempt to curtail that agreement, or others for that matter.

What I pushed the Saudis on was we needed a U.S.-Saudi Arabia pol-mil discussion that included DoD and State, to talk about what their requirements and capabilities are, to make sure the stuff they want to buy from us makes sense — in terms of their requirements and that they have the capability to both train and to use them.

Because the U.S. must maintain Israel's qualitative military edge, t here are sensitivities about certain platforms. A re there are more capabilities Saudi Arabia is interested in, the F-35 for example? 

There is interest, but that's why I recommended this dialogue. They're at a time in their economy where they're running deficits, they're having to tap into their reserves. There's no idea that oil prices are going to rebound back. And so, the question is, do you want to put your money in it. For the first time, their education budget really grew much more so than their defense budget. So, they are beginning to align their priorities to this economic transformation plan. It's really about the needs of the people: employment, jobs, and opportunities for people in Saudi Arabia. Less so about building a stronger defense system.

The U.S. wanted Saudi Arabia and other countries in the region to be more active against the Islamic State. Were there different priorities?

They were more than willing to be more aggressive and to help more in the fight against ISIL. The deputy crown prince offered to put an Islamic force together, recruited Muslim countries to be part of this and provide forces. But we were not ready to take on something like an Islamic force. The training, the complexity was beyond what we were interested in addressing. There were meetings with them, but it was something that really was not realistic, given the other challenges in the region.

Is there any leftover business from the Obama administration for President Trump to address, maybe things he might be less reluctant to do than Obama? 

There were certainly issues with President Obama. His article in the Atlantic, they were not happy with that. It reflected an attitude about Saudi Arabia and about Arabs that they felt uneasy about. There was always a sense of an attitude there, the president wanted not to engage, was not as supportive. On Syria, they wanted the United States really engaged, for what I think are good reasons. 

What they see with Trump is two sides. One is very supportive of Israel and talks about moving the embassy to Jerusalem — which is not necessarily a big problem. The ban on Muslim countries, Saudi Arabia wasn't included in that, so they feel that they're in a different position than other countries. I do feel they like [Trump's] rhetoric on Iran. But how that's going to develop, as far as the fight in Yemen and ISIL, that's going to be the test. It's a little early, so they're still trying to assess.

Probably the most crucial relationship that we have with Saudi Arabia is the counter-terrorism and intel relationship. They are very good, very sophisticated, and they have really worked with us and we have worked with them extensively for shutting off completely the valve of money to terrorist organizations from Saudi Arabia. The crown prince, his forces, his people, everything, are very formidable. One question, as the previous administration was leaving was, "Will the Trump administration maintain this crucial partnership?"

The Obama administration released the long-classified 28 pages of the official congressional report on the 9/11 attacks, which concerned alleged ties between the Saudi government and 9/11 hijackers. Did that, JASTA and Congress's pushback on the Yemen campaign damage the U.S.-Saudi relationship?

As an ambassador, one thing I did not understand was why the White House did not come address JASTA much earlier, much more aggressively than it did. Not because of Saudi Arabia. I think the president did everything he needed to on Saudi Arabia with respect of the 28 pages, the veto and everything else. But the biggest problem for me was not Saudi Arabia, but how does [potential reciprocal action on JASTA] affect our sovereignty, our people, our military, our contractors overseas and other places.

A lot has been made in the United States about the Trump administration's perceived Muslim ban. Is it damaging relationships in the Middle East, in the Muslim world, or is the backlash mainly here?

That's a really good question. I think that most of the fallout is here. The countries that the president identifies are countries that have big problems with terrorists operating inside their borders. I think it's that [where] the administration has failed is to explain. 

An executive order, I think, is the wrong way to govern a country. We're a representative democracy. And with it, there needed to be a much more in-depth explanation of how it's to be implemented, and its purpose, which they didn't do.

If this is an attempt to make this process more robust, to do more due diligence, to hold people more accountable for the visas we grant, that should have been explained up front, I think most Americans would have thought it was important to do.

How did some of the campaign rhetoric impact defense ties overseas? Will majority-Muslim countries overseas recoil over Trump's election and his perceived Muslim ban?

It hasn't been a ban on Muslims. It has not. You take countries like Pakistan, Saudi Arabia, all the Asian countries — Indonesia, we didn't ban them and they're one of the largest Muslim countries in the world. So, it's not a ban on Muslims, and that's why I think he needed to explain it better.

Otherwise, I think there's quite a lot of hope that this new secretary of state — given he's a been a businessman, given he's been an oilman, that he's been working in the Middle East, working with countries in Asia, China, knows Russia, that he has a business management portfolio — that this would be a new way to approach these issues. The Saudis are thinking, this may be good for us, we need to pay more attention to our economic relationship. We have this transformation plan. It's about, how do we transform Saudi Arabia? We need more foreign investment. I think there's hopefulness there.

Defense Secretary James Mattis is somebody they also feel is experienced in the region and is sympathetic to the issues they face. I see them feeling very good about him, that maybe this individual will bring something new and positive to the relationship.

This interview was edited and condensed.

Twitter: @ReporterJoe